Method and apparatus for making color cathode ray tube

ABSTRACT

IN ORDER TO AVOID UNDESIRABLE LOSS OF COLOR DISPARITY IN THE PERFORMANCE OF A CATHODE RAY TUBE OF THE MULTIBEAM, MULTI-COLOR, SHADOW MASK TYPE, IT IS PREFERABLE TO LIMIT THE DIAMETER OF EACH COLOR DOT ON THE PHOSPHOR SCREEN. IN THE PROCESS OF FIXING PHOSPHOR DOTS ON A PHOSPHOR SCREEN INSIDE A FACE PLATE OF THE CATHODE RAY TUBE DURING THE FABRICATION THEREOF, A SPECIALLY SHAPED LIGHTEMITTING SOURCE IS EMPLOYED, IN ORDER TO LIMIT THE DIAMETER OF EACH PHOSPHOR DOT TO NO LARGER THAN THAT OF EACH APERTURE IN THE SHADOW MASK THROUGH WHICH ULTRAVIOLET RAYS STRIKE THE SCREEN TO FIX SAID DOT. SAID LIGHT-EMITTING SOURCE IS FORMED TO HAVE A LIGHT-EMITTING FACE WHICH IS SHAPED IN THE FORM OF A RING DISPOSED IN PARALLEL RELATIONSHIP WITH THE FACE PLATE AND WHICH IS FORMED ON THE TOP OF A LIGHT CONDUIT MADE OF A TRANSPARENT MATERIAL.

1973 CHIKAYOSHI NINAGAWA ETA!- 3,754,366

METHOD AND APPARATUS FOR MAKING COLOR CATHODE RAY TUBE Filed 001;. 30, 1970 2 4 fl p/0P APT) INVENTORS S\-\\GE4P\ RsmlFWu a MasRmRo OKHMOTO cmxmosw m N HCrHWH,

BY RM, 35AM ATTORNEYS United States Patent O U.S. Cl. 11733.5 CM 2 Claims ABSTRACT OF THE DISCLOSURE In order to avoid undesirable loss of color disparity in the performance of a cathode ray tube of the multibeam, multi-color, shadow mask type, it is preferable to limit the diameter of each color dot on the phosphor screen. In the process of fixing phosphor dots on a phosphor screen inside a face plate of the cathode ray tube during the fabrication thereof, a specially shaped lightemitting source is employed, in order to limit the diameter of each phosphor dot to no larger than that of each aperture in the shadow mask through which ultraviolet rays strike the screen to fix said dot. Said light-emitting source is formed to have a light-emitting face which is shaped in the form of a ring disposed in parallel relationship with the face plate and which is formed on the top of a light conduit made of a transparent material.

This invention relates to an improvement in a method and apparatus for forming phosphor dots on the screen of a color cathode ray tube for use in a color television set.

BACKGROUND OF THE INVENTION Hitherto, in a color cathode ray tube of the multibeam, multi-color, shadow mask type, phosphor dots have been fixed in a particular array on the face of the tube by means of the photo-chemical fixing method, wherein an emulsion containng phosphor and photo-fixing agent is irradiated by ultraviolet ray beams transmitted through apertures in the shadow mask, as explained in U.S. Pat. No. 285,935 to D. W. Epstein et al. According to this conventional method, the phosphor dot is inevitably formed with a diameter larger than that of the aperture in the shadow mask through which ultraviolet rays strike the screen to fix said dot. Accordingly, the phosphor dots are of sufiicient size to be tangent to each other. On account of this close proximity of the phosphor dots, even a slight drifting of the path of the electron beam may cause undesirable striking of adjacent color dots, resulting in lack of color disparity or inaccurate color expression.

in order to solve this problem, it is preferable to limit the diameter of each color dot so that the color dots are arranged with suitable spaces therebetween. However, such limiting of the diameter of these very small dots is very difiicult, especially with presently known methods of fabrication.

In the conventional method of photo-chemical fixing of phosphor dots, a light source having a hemispherical light-emitting surface is employed to emit photo-chemical rays, such as ultraviolet rays, onto a screen coated with a phosphor slurry. Details of such process are described referring to FIG. 1 and FIG. 2, wherein a phosphor-containing emulsion 9 applied on the inside face of the face plate 8 of a color cathode ray tube is struck by photo-chemical rays, such as ultraviolet rays 4', which are generated by an ultraviolet lamp 2 in a light box 1. The ultraviolet rays are then transmitted through the transparent light conduit 3 in the light box 1 and are Patented Oct. 9, 1973 emitted from the hemispherical emitting face 4, so as to pass through the correcting lens 5 where they are suitably refracted. Finally, the rays pass through apertures 7 in the shadow mask 6, located at a specified distance inside the face plate 8, and strike the emulsion 9 in a pattern of dots. Details concerning the distribution of the ultraviolet rays on the emulsion 9, due to their passage through one of said apertures from said lightemitting surface, are shown in FIG. 2, wherein dimensions of parts and their arrangements as shown are exaggerated for the sake of easy understanding, and the correction lens 5 is omitted since it has no substantial influence over the distribution of received rays.

As can be understood from FIG. 2, the emulsion 9 is struck by the ultraviolet rays 4' coming through each aperture 7, and the energy of the received rays is distributed in the form of a circular truncated cone, which cone is composed of a central part 10 and a surrounding outskirt part 10', the latter having an outer diameter larger than that of the aperture 7. The parts of the emulsion 9 thus exposed to the ultraviolet rays 4 are hardened by known photo-chemical reaction, and such hardness depends on the energy of the received rays. Accordingly, the central part of the exposed area acquires the greatest hardness and the hardness gradually diminishes towards the outskirt part of the dot until it becomes zero at the outmost boundary.

The exposed emulsion is then developed by being washed with a hot spray or a shower of washing liquid so as to retain dots at the exposed areas. In order to reduce the diameter of each dot, attempts have been made to remove the portion at the outskirt of each exposed area, where the hardness is comparable low, by applying a rather strong hot shower or spray thereto. However, since the portion at the outskirt of the exposed area has no critical zone where the hardness of the exposed emulsion changes abruptly, only the outermost part of the portion at the outskirt can be removed through the developing process, and in addition, the dots sometimes acquire a non-circular shape during such a process. Thus, it has hitherto been diflicult to obtain phosphor dots with a diameter smaller than that of the apertures on the screen.

SUMMARY OF THE PRESENT INVENTION The principal object of the invention is to provide an improved color cathode ray tube, wherein picture dots are arranged with suitable spaces therebetween, in order to avoid undesirable loss of color disparity, due to undesirable drifting of the electron beam path while in use, and to assure more accurate color expression.

Another object of the present invention is to provide an improved color cathode ray tube having a wider tolerance in dynamic convergence of a set of electron beams.

In the method of the present invention, a specially shaped light emitting source is employed in order to limit the diameter of each phosphor dot to a size no larger than that of each aperture in the mask through which ultraviolet rays strike the screen to fix the phosphor dot.

BRIEF DESCRIPTION OF THE DRAWING Further objects and advantages of the present invention will be best understood from the following detailed description when read in conjunction with the accompanying drawings, in which:

FIG. 1 is an elevation, in section, of a photographic lighthouse having an optical system for use in making color cathode ray tube, and a shadow mask and a screen plate of a color cathode ray tube set up thereon;

FIG. 2 is a partially schematic sectional view of the aforementioned prior art optical arrangement including a face plate, phosphor-containing emulsion, shadow mask, hemispherical faced light source top, together with a curve showing the distribution of the rays received by the emulsion;

FIG. 3(a) is a sectional view of a light conduit for a light source in accordance with the present invention;

FIG. 3(b) is a perspective view of the light conduit shown in FIG. 3 (a); and

FIG. 4 is a schematic view of the present invention, in section, including a face plate, phosphor-containing emulsion, shadow mask and a specially designed light-source, together with a curve showing the distribution of the rays received by the emulsion.

In FIGS. 2 and 4, the dimensions of the parts and arra-ngement thereof are not shown in exact proportion, but are exaggerated for the sake of easy understanding, and the correction lens is omitted since it has no substantial influence over the distribution of received rays.

DETAILED DESCRIPTION OF THE INVENTION In FIGS. 3(a) and 3(b), the light conduit 11 of the light source of the present invention is made of a heatresistive, transparent material of comparatively high refractive index, for instance, quartz glass, and is so shaped to provide a ring-shaped light-emitting face 12 on the top of a generally cylindrical or frusto-conical part and a light-intake face 13 at the bottom thereof. The ring-shaped light-emitting face 12 is positioned substantially in parallel with the inner face of the face plate 8 which is substantially in parallel with the shadow mask 6. The concave inner conical depression 11' in the ring-shaped light-emitting face 12 is formed by a surface having a suitable conical angle so as to exclude emission of light therefrom. Similarly to what is shown in FIG. 1, a face plate 8 and a shadow mask 6 are mounted in a lighthouse 1' which contains a correction lens and a light box 1 comprising a lamp 2 and a light conduit 3. Phosphor-containing emulsion 9 applied on the inside face of the face plate 8 is struck by photo-chemical rays, such as ultraviolet rays 12', which are generated by the lamp 2 in the light box 1, and then are conducted through the transparent light conduit 11 in the light box 1, so as to be emitted from the emitting face 12. Next, the rays are suitably refracted by the correcting lens 5, and finally pass through aperture 7 in the shadow mask 6 located inside the face plate 8.

According to the present invention, the photo-chemical rays, such as ultraviolet rays 12', are emitted only from the ring-shaped light-emitting face 12, as shown in FIG. 4. As can be understood from FIG. 4, the emulsion 9 is struck by the ultraviolet rays 12' coming through each aperture 7, and the energy of the received rays is distributed in a pattern resembling a two-storied circular truncated cone, which cone is composed of a higher central part 14 and a surrounding lower outskirt part 14'. It is a feature of the present process that there is a considerable difference between the light energy level of the central part 14 and that of the outskirt part 14, separated by a distinctive circular edge 14" inbetween. The higher central part 14 has a diameter considerably smaller than that of the lower outskirt part 14'. Moreover, the diameter of the central part 14 is made smaller when the diameter of the ring-shaped emitting face is made larger. The parts of the emulsion 9 thus exposed to the ultraviolet rays 12' are hardened by known photochemical reaction, and such hardness depends on the energy of the received rays. Accordingly, the central part of the exposed area acquires the greater hardness and the hardness of the outskirt part of the dot is distinctly less than that of the central area.

The exposed emulsion 9 is then developed by being washed with a hot spray or shower of washing liquid so as to retain dots at the exposed areas. During such de- 4 1 velopment, the less-hardened outskirt part of the exposed dot on the emulsion-9 is selectively removed.

Since the exposed area has a discontinuity at the circular edge 14", the outskirt part is removed smoothly, and the central part having a higher hardness is retained in a satisfactorily clear circle matching the circular edge 14". Thus, phosphor dots having a diameter smaller than that of the apertures, as well as a smooth round periphery, separated by considerable space inbetween, are obtained.

Since the diameter of the phosphor dots can be made smaller than that of the aperture of the shadow mask, according to the present invention, it is possible to make the aperture larger than the conventional ones so as to make the picture on the screen face brighter without creating a crowding between adjacent phosphor dots and consquent undesirable loss of color disparity and unwanted mixing of the emitted colors.

While we have shown and described one embodiment in accordance with the present invention, it is understood that the same is not limited thereto but is susceptible of numerous changes and modifications as known to a person skilled in the art, and we therefore do not wish to be limited to the details shown and described herein but intend to cover all such changes and modifications as are obvious to one of ordinary skill in the art.

We claim:

1. Method of making a color screen for a cathode ray tube comprising the steps of:

coating one inside face of the cathode ray tube with a photochemically hardenable phosphor-containing emulsion, irradiating said coating through an apertured mask with photochemical rays emitted from a lamp source having a ring-shaped light emitting face positioned substantially parallel with said coating, said apertured mask being spaced away from said inside face and the diameter of said ring-shaped light emitting face being larger than that of the apertures in said apertured mask so as to create a ray energy distribution pattern resembling a two-storied circular truncated cone, controlling said irradiating so as to achieve greater hardness of the coating areas exposed to the energy represented by the portion above the truncation of said truncated-cone energy distribution pattern than of the coating areas exposed to the energy represented by the'portion below the truncation of said truncated-cone energy distribution pattern, and

washing said irradiated coating to remove all the areas thereof except said areas exposed to the energy represented by the portion above said truncation,

whereby the phosphor dots of the color screen are of a smaller diameter than the diameter of the apertures of said apertured mask. r

2. Method as defined in claim 1, wherein said photochemical rays are ultraviolet rays.

References Cited UNITED STATES PATENTS 3,615,460 10/1971 Lange 96.-36.1

3,380,354 4/1968 Thornton -1 R 3,109,116 10/1963 Epstein et al. 313-92 BX 3,499,372 3/1970 Staunton 313 92 BX 3,581,136 5/1971 Staunton 313 92 BX FOREIGN PATENTS 1,168,826 10/1969 Great Britain ALFRED L. LEAVITT, Primary Examiner US. Cl. X.R. 

